no earthly reason
No conceivable or practical reason. You had all day, so there's no earthly reason you couldn't have finished this on time! It's very sweet that you bought me a laptop, dear, but I'm 85—I have no earthly reason to use one of those things!
no earthly reason
no conceivable reason. There is no earthly reason for your rude behavior. I can think of no earthly reason why the repairs should cost so much.
no earthly reason
n. no conceivable reason. I can think of no earthly reason why the repairs should cost so much.